tests: spi: convert nucleo_g474re to define prescaler in dt

As the nucleo_g474re was converted to define clock properties
in devicetree, this commit makes the ncecessary changes such that
the ncecessary test properties are still applied.

Signed-off-by: Thomas Stranger <thomas.stranger@outlook.com>

tests/drivers/spi/spi_loopback/boards/nucleo_g474re.overlay

@@ -30,3 +30,11 @@
 &dmamux1 {
 	status = "okay";
 };
+
+&rcc {
+	/*
+	 * Reduce bus clock speed to be able to reach
+	 * SPI_LOOPBACK_SLOW_FREQ = 500000 with max prescaler 256
+	 */
+	apb2-prescaler = <2>;
+};